CDMA (Code Division Multiple Access) systems allow multiple users to share a common frequency and a common time channel by using encoded modulation. Some CDMA systems that could be mentioned are CDMA 2000, W-CDMA (Wide Band CDMA) and the IS-95 standard.
Within CDMA systems, those that use separate frequencies for the transmission and the reception (CDMA-FDD system), and those that use a common frequency for the transmission and the reception, but separate time domains for the transmission and the reception (CDMA-TDD system), may be further distinguished.
As opposed to communication systems having a constant envelope modulation, such as the GSM system, CDMA systems use a modulated signal that has a modulation with a varying envelope. In other words, such a signal is both phase and amplitude modulated, and the amplitude of the modulation is not constant.
In remote terminals, such as cellular mobile telephones, the transmission power amplifier stage should have a wide range of radio frequency power operations. Also, notably in CDMA-FDD systems, the power amplifier is in continuous operation during communications.
More precisely, the transmission power delivered by the power amplifier can vary over a predetermined power range, typically from −50 dBm to 24 dBm in steps of 1 dB for third generation mobile telephones. The transmission power is adjusted, over this power range, as a function of power information received periodically by the telephone and which comes from the base station.
Furthermore, each time there is a request to modify the transmission power by a base station, this modification should be carried out within a short time, typically 50 μs. To verify the transmission power, this should be detected at the output of the system, and when the signal has a varying envelope the detector output signal exhibits ripple that can be very significant and be situated in a frequency range from 0 to 4 MHz in W-CDMA systems. It is then necessary to filter this output signal. However, this filtering process requires a very long time constant which is incompatible with the time for establishing the requested transmission power.